[ ] — Enclose / Literal / Index / AxisStandalone [Y] encloses Y, including functions.
[1 2 3] ⍝ ⊂1 2 3
f←[+] ⋄ 2(↑f)3 ⍝ 5
After a dyadic operator, brackets supply an enclosed right operand.
(1∘+)⍣[3]⊢0 ⍝ 0 1 2 3
⊢⍣[≡]⊢4 ⍝ 4 4
[A ⋄ B] assembles cells with fill. Newlines also separate cells.
[1 2 ⋄ 3 4] ⍝ 2 2⍴1 2 3 4
[1 2 ⋄ 3] ⍝ 2 2⍴1 2 3 0
After an array, brackets index from 1. Semicolons separate axes; an empty field selects the whole axis.
v←10 20 30
v[3 1] ⍝ 30 10
v[2] ⍝ 20
v[⊂2] ⍝ ⊂20
v[,2] ⍝ ,20
m←2 3⍴⍳6 ⋄ m[;2] ⍝ 2 5
Complete atomic indices retrieve the stored value. Array indices supply a result frame; omitted axes retain the remaining cell axes.
After a function, brackets qualify axes.
+/[1]2 3⍴⍳6 ⍝ 5 7 9
See axis rules, assignment and Squad.